Network systems enable communication of messages among computer systems. For example, network systems enable communication of resources over the Internet. Increases in computer and Internet usage have resulted in an increased number of resources being exchanged, which has caused network resources to become increasingly taxed and difficult to operate and maintain. To complicate matters, resources may be embedded in messages being exchanged over a network, which may lead to the dedication of additional network resources to the communication and storage of particular resources. In fact, popular resources (e.g., pictures) may be attached or embedded and sent numerous times in messages from a single source or from subsequent recipients of the messages.